The present invention relates to propping agents for use in hydraulic fracturing of subterranean formations surrounding oil wells, gas wells, and similar boreholes.
Hydraulic fracturing is a well stimulation technique designed to increase the productivity of a well by creating highly conductive fractures or channels in the producing formation surrounding the well. The process normally involves two steps. First a fluid is injected into the well at a sufficient rate and pressure to rupture the formation thereby creating a crack (fracture) in the reservoir rock. Thereafter a particulate material (propping agent) is placed into the formation to "prop" open the fracture.
In order for well stimulation to occur, the propping agent must have sufficient mechanical strength to withstand the closure stresses exerted by the earth. If the propping agent is not strong enough to resist the earth's closure stresses, then the propping agent will tend to disintegrate thereby reducing the permeability of the propped fracture.
On the other hand, the propping agent must also be inexpensive since large volumes of propping agent are used in a well stimulation treatment. For example, it takes 135,520 pounds of sintered bauxite propping agent to fill a 968 ft.sup.3 fracture. McDaniel et al., "The Effect of Various Proppants and Proppant Mixtures on Fracture Permeability," SOCIETY OF PETROLEUM ENGINEERS OF AIME, AIME Paper No. SPE 7573 (1978) at p. 4 (McDaniel et al.)
Because of its low cost, relative abundance and low density, sand is the ideal propping agent for hydraulic fracturing of low closure stress (4,000 psi or less) formations. While specially screened (usually 20-40 mesh) high grade sand (e.g., Ottawa sand) can be used with higher closure stress formations, performance drops off drastically as stress increases, particularly above 8,000 psi. At stresses of 10,000 psi and above even the highest grade sand is inadequate.
Heretofore, the only propping agents known to be able to withstand closure pressures of 10,000 psi or greater was a high density sintered bauxite propping agent. Cooke, "Hydraulic Fracturing with a High-Strength Proppant", SOCIETY OF PETROLEUM ENGINEERS OF AIME, AIME Paper No. SPE 6213 (1976); Jones et al., "Light Weight Proppants for Deep Gas Well Stimulation", submitted to Bartlesville Energy Technology Center by Terra Tek, Inc., under Government Contract # DE-AC19-79BC10038 (June 1980) (Terra Tek paper); McDaniel et al. Described in U.S. Pat. No. 4,068,718 (Cooke et al.), this high strength propping agent consists of sintered bauxite particles having a specific gravity greater than 3.4. High density is described as a critical feature of this propping agent. According to Cooke et al., the high density is necessary to attain high strength and resist fragmentation under high stress levels. They explain that permeability drops off significantly when low density particles (specific gravity below 3.4) are used. (Col. 6, lines 6-35.)
High grade alumina propping agents were also reported to be able to withstand closure pressures of 10,000 psi in U.S. Pat. No. 3,976,138 (Colpoys et al.). This, however, has not been confirmed by independent investigators. In any event, the Colpoys et al. propping agent also consists of high density particles, i.e., specific gravities of 3.40 and greater. Although Colpoys et al. describe a lower density low grade alumina propping agent as well, they do not report that this less preferred propping agent is able to withstand such severe conditions.
While the sintered bauxite propping agent is advantageous in that it can withstand closure pressures of 10,000 psi or greater, there are certain disadvantages associated with its use. On account of its high-density, it requires high viscosity fracturing fluids and/or high pumping rates along with low proppant concentration. This makes fracture control and high conductivity fractures more difficult to obtain. (See Terra Tek paper at p. 3.) Since the Colpoys et al. propping agent has a similar high density, the same problems would be expected to be associated with its use. Additionally, sintered bauxite is relatively expensive. The cost per pound of bauxite is ten to fifteen times that of sand. (See Terra Tek paper at pp. 2-3 and McDaniel et al. at p. 4.) Accordingly, there has been a definite need for a lower density and less expensive propping agent having the mechanical strength to withstand closure pressures of 10,000 psi or greater.